Consumers identify whiteness and cleanliness of fresh white button mushrooms (Agaricus bisporus) as the principal factors determining the quality thereof (McConnell, 1991; Beelman, 1987; Schisler, 1983; Barendse, 1984; Wuest, 1981). Consumers prefer to purchase mushrooms which are bright white and free of casing material, compost, or other unwanted particulate contaminants clinging to the surfaces thereof (McConnell, 1991).
Commercial mushroom cultivation practices, typically growing mushrooms in straw-bedded horse manure compost covered with a fine layer of peat or other "casing material," yields mushrooms with unwanted particulate contaminants clinging to the mushroom cap and other surfaces, giving an undesirable appearance (McConnell, 1991). Moreover, mushrooms are typically harvested by hand, introducing a source of contamination with fluorescent pseudomonads and other spoilage organisms, leading to accelerated tissue decay and discoloration (McConnell, 1991).
Mushroom discoloration (browning and purple blotch) occurs when a polyphenol oxidase enzyme (tyrosinase), which naturally occurs at high levels in mushroom cap cuticle (surface) tissue, interacts with phenolic substrates, also naturally occurring in mushroom tissue, to produce the brown pigment melanin. In healthy, intact mushroom tissue, the enzyme and its substrates are located in separate subcellular compartments, and are therefore prevented from reacting to form colored pigments. Unfortunately, mushroom tissue is highly susceptible to damage by bacterial action or by physical handling, and this damage allows the browning enzyme and its substrates to interact, resulting in unwanted color changes in the mushroom tissue.
It would be highly desirable, therefore, to provide a commercial, toxicologically acceptable preservative treatment to prevent bacterial damage to mushroom tissue, indirectly preventing discoloration, and to inhibit directly the polyphenol oxidase-mediated browning reaction. Moreover, it would be especially desirable to introduce these preservatives to mushrooms in the form of a spray or wash which would remove compost, casing material, and other unwanted particulate material cling to mushroom surfaces.
Prior to 1986, aqueous solutions of sulfite, particularly sodium metabisulfite, were used to wash mushrooms for the purpose of removing unwanted particulate matter, and to enhance mushroom whiteness. In 1986, however, the U.S. FDA banned the application of sulfite compounds to fresh mushrooms, due to severe allergic reactions to sulfites among certain asthmatics.
Following the ban on sulfite compounds for processing of fresh mushrooms, there have been several efforts to develop wash solutions for use as a suitable replacement for sulfites. While sulfite treatment yields mushrooms of excellent initial whiteness and overall quality, it does not inhibit the growth of spoilage bacteria. Therefore, the quality improvement brought about by sulfite use is transitory. After 3 days of refrigerated storage, bacterial decay of sulfited mushrooms becomes evident. Traditionally, this was not a concern to mushroom growers, because sulfite washes were inexpensive, effective at removing unwanted particulates, and gave excellent initial quality.
The banning of sulfite washes, however, gave researchers incentive not only to find a suitable sulfite replacement, but also to improve upon sulfite washes by developing a preservative treatment which would extend washed mushroom shelf life beyond that attainable by sulfiting, and which would improve storage quality over that of sulfited mushrooms. McConnell (1991) developed an aqueous preservative wash solution containing 10,000 parts per million (ppm) hydrogen peroxide and 1000 ppm calcium disodium EDTA. The hydrogen peroxide serves as an antimicrobial agent, while EDTA enhances antimicrobial activity and directly interferes with the enzymatic browning reactions. Copper is a functional cofactor of the mushroom browning enzyme, tyrosinase, and tyrosinase activity is dependent upon copper availability. EDTA binds copper more readily than does tyrosinase, thereby sequestering copper and reducing tyrosinase activity and associated discoloration of mushroom tissue.
Hydrogen peroxide acts as a bactericide by causing oxidative damage to DNA and other cellular constituents. Sapers (1994) adapted McConnell's (1991) hydrogen peroxide treatment, incorporating hydrogen peroxide into a two-stage mushroom wash, employing 10,000 ppm hydrogen peroxide in the first stage and 2.25% or 4.5% sodium erythorbate, 0.2% cysteine-HCL, and 500 ppm or 1000 ppm EDTA in aqueous solution in the second stage. Hydrogen peroxide treatments typically yielded mushrooms nearly as white as sulfited mushrooms initially, and whiteness surpassed that of sulfited mushrooms after 1-2 days of storage at 12.degree. C., and shelf life was dramatically improved (McConnell, 1991). Hydrogen peroxide, however, is not currently approved for treatment of fresh produce. More efficacy and safety data are required. Moreover, as the browning reaction itself is oxidative, it would be advantageous to employ a non-oxidative agent, rather than a strong oxidizer such as hydrogen peroxide, for controlling bacterial growth.